console.log(typeof 10) //number
console.log(typeof '10') //string
let a = 10, b = '10'
//Number(b) 将b变量的类型转换为number
console.log(a + 10, b + 10, Number(b) + 10, parseInt(b) + 20)

let i = 9
console.log(i.toString().padStart(3, '0')) //3
// 003
if (i < 10) {
    i = '00' + i
} else if (i < 100) {
    i = '0' + i
}
console.log(i)
